The present invention relates generally to bi-directional overrunning clutch assemblies and, more particularly, to an actively controlled, multi-mode, bi-directional overrunning clutch assembly used in a four-wheel drive transfer case.
Four-wheel drive vehicles are in great demand since consumers desire the enhanced on-road and off-road traction control they provide. In many four-wheel drive vehicles, a transfer case is installed in the drivetrain and is normally operable to deliver drive torque to the primary driveline for establishing a two-wheel drive mode. The transfer case is further equipped with a clutch assembly that can be selectively or automatically actuated to transfer drive torque to the secondary driveline for establishing a four-wheel drive mode. These xe2x80x9cmodexe2x80x9d clutch assemblies can range from a simple dog clutch that is operable for mechanically shifting between the two-wheel drive mode and a xe2x80x9clockedxe2x80x9d (i.e., part-time) four-wheel drive mode to a sophisticated automatically-actuated multi-plate clutch for providing an xe2x80x9con-demandxe2x80x9d four-wheel drive mode.
On-demand four-wheel drive systems are able to provide enhanced traction and stability control and improved operator convenience since the drive torque is transferred to the secondary driveline automatically in response to lost traction of the primary driveline. An example of passively-controlled on-demand transfer case is shown in U.S. Pat. No. 5,704,863 where the amount of drive torque transferred through a pump-actuated clutch pack is regulated as a function of the interaxle speed differential. In contrast, actively-controlled on-demand transfer cases include a clutch actuator that is adaptively controlled by an electronic control unit in response to instantaneous vehicular operating characteristics detected by a plurality of vehicle sensors. U.S. Pat. Nos. 4,874,056, 5,363,938 and 5,407,024 disclose various examples of adaptive on-demand four-wheel drive systems.
Due to the cost and complexity associated with such actively-controlled on-demand clutch control systems, recent efforts have been directed to the use of overrunning clutches that can be controlled to provide various operating modes. For example, U.S. Pat. No. 5,993,592 illustrates a pawl-type controllable overrunning clutch assembly installed in a transfer case and which can be shifted between various drive modes. U.S. Pat. No. 6,092,635 discloses a hydraulically-actuated multi-function controllable overrunning clutch assembly noted to be operable in vehicular power transmission mechanisms. Likewise, U.S. Pat. Nos. 5,924,510, 5,951,428, 6,123,183, and 6,132,332 each disclose a controllable multi-mode overrunning clutch installed in a transfer case and actuated using an electromagnetic clutch.
However, controllable overrunning clutch assemblies do not always provide predictable vehicle handling characteristics or the required durability necessary for use in modern four-wheel drive vehicles. Thus, a need exists to continue development of controllable bi-directional overrunning clutch assemblies which provide improved structure, robust operation, and reduced packaging for use in on-demand transfer cases.
The present invention is directed to a controllable, multi-mode, bi-directional overrunning clutch assembly and a shift system adapted for use in a transfer case for transferring drive torque from a primary output shaft to a secondary output shaft so as to establish a four-wheel drive mode. The clutch assembly includes a first ring journalled on a first rotary member, a second ring fixed to a second rotary member, and a plurality of rollers disposed in opposed cam tracks formed between the first and second rings. The first ring is split to define an actuation channel between its end segments. A cam rod is moveable between positions engaged with and released from one or both end segments of the split first ring. The shift system includes a mode fork which controls movement of the cam rod for establishing four distinct operational modes; on-demand 4WD-forward mode; an on-demand 4WD-reverse mode; a part-time 4WD mode (both directions); and a 2WD mode (both directions).
The transfer case of the present invention also includes a two-speed gearset and a range sleeve that is moveable for establishing high and low-range drive connections. In such two-speed transfer cases, the shift system also functions to coordinate movement of the cam rod and the range sleeve to establish various combinations of speed ranges and drive modes.
In accordance with one embodiment of the present invention, the first ring is journalled on the secondary output shaft and the second ring is fixed to a rotary compound of a transfer assembly driven by the primary output shaft. Thus, the invention provides for installing the controllable, multi-mode, bi-directional overrunning clutch in association with the front output shaft to permit significant axial length reductions for the transfer case.
Thus, it is an object of the present invention to provide an on-demand transfer case equipped with a controllable, multi-mode, bi-directional overrunning clutch that advances the state of the four-wheel drive technology.
It is a further object of the present invention to provide a power-operated actuator for controlling shifting of the clutch assembly between its distinct modes in response to various sensor signals received by a controller unit.
Further objects, advantages and features of the present invention will become readily apparent to those skilled in the art by studying the following description of the preferred embodiment in conjunction with the appended drawings which are intended to set forth the best mode currently contemplated for carrying out the present invention.